The invention concerns a motor-driven servo steering system for a motor vehicle.
Prior art vehicles are known which are controlled through one or several steerable axles, wherein the driver transmits the desired directional change to the steerable wheels by actuation of the steering wheel. Typically, a hydraulically operated servo steering is currently used which supports or aids the steering force of the driver, especially with heavy vehicles.
Electric motors are also used for boosting the steering force of the driver, for example as described from the German patent disclosure 37 09 590. Compared to hydraulic systems, the use of electromotors for boosting the steering force enables in a simple way greater variation options. Advantageously, the degree of steering boosting is selected depending on the traveling speed of the vehicle.
Besides transmitting the steering desired by the driver, the vehicle steering systems may also be affected so that an increase in road stability is achieved. Specifically, rolling and pitching motions of the vehicle may be minimized by steering motions. For that purpose, the movements of the vehicle are monitored by various sensors, and the information on the vehicle movement picked up from the sensors are processed by a computer. According to these signals, the steering systems of the vehicle are actuated to improve the vehicle's ability to handle the road, i.e., road stability.
To increase the road stability of a vehicle it is thus necessary to combine the desired steering with the steering instructions of a computer system monitoring the vehicle movements. The turning angle of the steerable wheels is defined as the sum of two angles. The one angle is determined by the driver by means of actuation of a steering wheel, while the other angle is chosen by the computer system monitoring the vehicle movements.
In the German patent disclosure 39 19 990, a hydraulic system aids the steering force of the driver. In addition to the hydraulic system, the steering instructions that depend on travel movements are caused by an electric motor. In a conventionally designed system aiding the steering force of the driver, as described in the German patent disclosure 37 09 590, there is always a fixed ratio given between the angle of the steering wheel actuated by the driver and the steering angle of the steerable wheel. Additionally in the system of the German patent disclosure 39 19 990, a turning of the steerable wheels is to a limited extent possible independently of the turning angle of the steering wheel actuated by the driver.
The system of the German patent document 39 19 990 has essentially two disadvantages:
1. The steerable wheels can only to a limited extent be selected independently of the turning of the steering wheel actuated by the driver for increasing the road stability.
2. This steering system requires two power sources, namely a hydraulic system for the driver and an electric motor for the computer.
British patent document 1 414 206 discloses a servo steering system wherein the steering force of the driver is aided by a hydraulic system and where an angular velocity of an electric motor is superimposed on the steering wheel angular velocity. The electric motor is controlled by an auxiliary system that senses vehicle movements such as lateral forces caused by side winds. However this system also requires two power sources.
What is needed is an optimized steering system with high inherent safety.